The use of medical implants is a multi-billion dollar world-wide industry and the development of new applications is increasing rapidly. Medical implants include, among others, pacemakers, stents, drug delivery devices, and artificial organs.
One continuing problem in the implantation of devices within the body is the reaction of the body to the implanted device. For example, the body often rejects foreign objects implanted within the body, causing undesirable side effects that can injure a patient or require the administration of oral or intravenous medications to prevent rejection of the implanted device. Also, the implantation of stents within an artery or vein is often complicated by restenosis, a complication caused by the recurrence of plaque especially after balloon angioplasty coupled with stent implantation. For example, see U.S. Pat. No. 5,834,419, which issued to McFadden, et al. on Nov. 10, 1998, and is herein incorporated by reference in its entirety. A medication can be included in an organic coating that inhibits rejection of a medical device or inhibits restenosis at the location of a stent. However, this solution requires a tough, well-adhered, smooth, thin and continuous organic coating on the surface of the device or stent that can hold the inhibitor on the surface and/or release the inhibitor over time from the stent.
The process of coating a three-dimensional shape, such as a stent, with a uniform, continuous and conformal coating is a difficult one that has not heretofore been completely solved. Frequently, the process is complicated by the delicacy, intricacy, and ultimately in-vitro use of these medical devices. For example, a cardiac stent is normally compressed during catheterization, and when the stent is in position for emplacement, the stent is expanded by a balloon or other means, which opens the previously blocked or partially blocked vessel. A coating on the stent must be able to conform to the three-dimensional shape of the stent without interfering with the expansion of the stent, and the coating must remain adhered to the surface of the stent and must be continuous and smooth following expansion of the stent.
Another application requiring high quality coatings is the fabrication of surface acoustic wave sensors (SAWS) for detection of volatile compounds. SAWS resonate in the megahertz range, usually using piezoelectric materials to create the resonance. For example, a thin, chemically reactive coating of a particular organic compound allows the sensor to capture from the surrounding environment molecules of certain hazardous compounds or molecules associated with the presence of hazardous compounds. The sensor acts as a resonating mass microbalance. For example, see U.S. Pat. No. 6,314,791 to Rapp, et al., issued Nov. 13, 2001, which is incorporated herein by reference in its entirety. The presence of additional molecules that are captured by the surface coating registers as a change in the sound propagation speed of the surface wave, which can detect very low concentrations of the hazardous compounds. Intrinsically, this application requires an exceptionally adherent, thin, and uniform organic film that was difficult, if not impossible, to produce by any previously known process.
Until now, no deposition process satisfactorily achieved all of these objectives. Surface tension effects during deposition of an organic film often preferentially forms a meniscus or webbing at the interstices (which have a large negative curvature) of a three-dimensional substrate, such as can be found in a stent or medical device. Directed spray of liquid or semi-liquid droplets on a surface causes shadowing effects, which cause uneven or non-continuous coatings on the surface of a three-dimensional substrate.
A CVD process for coating a substrate using a liquid delivery System with an ultrasonic nozzle was disclosed in U.S. Pat. No. 5,451,260, which was filed on Apr. 15, 1994 and issued on Sep. 19, 1995, and is incorporated herein by reference in its entirety. This process produces a fine mist of very small droplets that rapidly evaporate in a vacuum chamber, such that only vapor comes in contact with the substrate. A uniform film then deposits on the substrate surface by a chemical vapor deposition process, whereby the vapor decomposes by pyrolysis, leaving a uniform metal oxide film on the surface. Although a uniform coating results on a flat surface, this process has the disadvantage of being a vapor deposition process, which can cause webbing at the interstices of a stent, for example. Furthermore, it does not allow for the deposition of a liquid that is not easily vaporized in a vacuum reaction chamber. Finally, it does not provide for pressure control during the drying of a liquid film on the surface of the substrate; therefore, vacuum levels sufficient to cause boiling on the surface of the substrate can cause an “orange peel” effect.
By “orange peer” the inventors mean that rapid volatilization of a solvent or other volatile compounds in a liquid film on the surface of a substrate can cause eruptions in the otherwise smooth and continuous coating. These eruptions are often not completely refilled by the surrounding liquid, leaving indentations on the surface that appear under magnification to resemble the irregular dimpling in the peel of an orange. If the coatings are required to be smooth, this dimpling is a cause for rejection of the coated device.